1. Field of the Invention
This invention relates to a printing apparatus for printing on recording media of varying thickness. More particularly, this invention relates to a printing apparatus in which a predetermined gap between a printing head and a platen can be easily set in accordance with the thickness of a recording medium to be used.
2. Description of the Related Art
A printing apparatus is required to optimally set a distance which is called a print gap, between an end part of a printing head and a surface of a printing medium, in order to obtain high quality printing on the recording medium. It is therefore desirable to preset the print gap in accordance with the thickness of the recording medium, such as a bank book, single layer paper, multi-carbon paper, or the like.
FIG. 14 is a front elevation view of a prior art printing apparatus. In FIG. 14, the printing apparatus includes a printing head 12, a platen 14, and a pressure sensor 16. The printing head 12 is positioned adjacent the platen 14, with an ink ribbon 18 and a head mask 20 having a print window 22 positioned therebetween. The printing head 12, ink ribbon 18 and head mask 20 are transferred along the platen 14 by a carriage (not shown in FIG. 14) to print out data on a recording medium 10.
The platen 14 includes cams 24a and 24b, cam followers 26a and 26b, a shaft 28, and springs 30a and 30b. The cam followers 26a and 26b are individually fixed to each side portion of the platen 14. The cams 24a and 24b respectively engage the cam followers 26a and 26b, and the cams 24a and 24b are connected to each other by the shaft 28. The springs 30a and 30b energize the platen 14 so the cam followers 26a and 26b are always in contact with the cams 24a and 24b, respectively. The shaft 28 is connected to a pulse motor 32. The pulse motor 32 and the pressure sensor 16 are coupled to a controller 34.
In FIG. 14, the printing medium 10 is fed to the printing area formed between the printing head 12 and the platen 14 by feed rollers (not shown in FIG. 14). Before the recording medium 10 reaches the platen 14, the platen 14 has been previously located at a lowered position. Therefore, the recording medium 10 is smoothly fed on the platen 14. After the recording medium 10 has reached the position of the platen 14, the pulse motor 32 is driven to rotate the cams 24a and 24b under control of the controller 34. As a result, the platen 14 is pushed upwardly by the cam followers 26a and 26b, and the recording medium 10 is positioned at the front surface of the head mask 20.
When the output signal of the pressure sensor 16 reaches a predetermined pressure value, the pulse motor 32 is driven in the reverse direction by a predetermined number of steps, so that the platen 14 is lowered. A proper gap is thereby set between the front surface of the head mask 20 and the surface of the recording medium 10, because the distance from the front surface of the head mask 20 to the end part of the printing head 18 is always kept at a predetermined value.
FIG. 15 is a side elevation view of another prior art printing apparatus. In FIG. 15, a printing head 40 is positioned against a platen 42 with an ink ribbon 44, a head mask 46 having a print window (not shown in FIG. 14), and a recording medium 48 therebetween. The platen 42 is supported by levers 50a and 50b and is upwardly energized by a spring 52 to push the recording medium 48 against the head mask 46. The movement of the platen 42 is regulated by a stopper 54. The print gap between the surface of the recording medium 48 and the end part of the printing head 40 is set because a gap from the front surface of the head mask 46 to the end part of the printing head 40, is set.
In FIG. 15, when a recording medium 48 has been fed to a printing area between the printing head 40 and the platen 42 by a feed roller (not shown in FIG. 15), the platen 42 is pushed downwardly by the force of the recording medium 48 which is fed by the feed roller, against the elastic force of the spring 52. The recording medium 48 is set to a printing position between the printing head 48 and the platen 42.
In the prior art apparatus of FIG. 14, because the pressure sensor 16 protrudes from the front surface of the head mask 20, the pressure sensor 16 tends to hook or snag the recording medium 10 during the printing operation, so that the recording medium 10 is shifted to an improper position. Also, because the pressure sensor 16 contacts the recording medium 10 during line feed operation, the recording medium 10 tends to be fed in an oblique direction, making it impossible to print out at the proper position on the recording medium 10. Moreover, since a surface of the pressure sensor 16 slides on the recording medium 10 during the printing and line feed operation, the surface of the pressure sensor 16 becomes worn, and the gap between the surface of the recording medium 10 and the head mask 20 is changed. Therefore, it is also impossible to maintain the desired gap for a long period of time.
On the other hand, in the prior art apparatus of FIG. 15, due to friction between the recording medium 48 and the head mask 46, the recording medium 48 can become jammed. In addition, where a pulse motor is used as a driving source for spacing of the printing head 40, synchronous operation of the pulse motor is disturbed by the friction, and asynchronous operation is generated. The asynchronous operation results in a problem that a printing position of a character to be printed out on a recording medium 48 is shifted to an improper position.